This invention relates to measuring the distance between two points within a body.
With respect to a human body, measurement errors as small as 1 mm can be significant. For example, a distance of 3.5 mm between the atlas and axis spinal bones may be normal while a distance of 4.5 mm may indicate atlanto-axial subluxation requiring further evaluation. And small changes in the distance over time may be important to diagnosis.
It is known to use a conventional ruler to measure distances directly on the X-ray film. Sometimes special rulers (having radio opaque material embedded in the index lines along the ruler) are placed at a location either between the X-ray source and the body, or between the body and the X-ray film. The X-ray film will then bear an image of the ruler's index lines that aid in measuring distances on the film.
Pelvimetry measuring devices have stable bases that rests on the same surface as the person's body. A ruler is attached to the base in a way that permits it to be adjusted in height and orientation to lie on top of the abdomen.
In typical X-ray techniques, distances on the image of the X-ray film are either enlarged or reduced relative to real distances in the body.
Some radio opaque rulers include a wooden base that slightly raises the scale purportedly to be in a similar plane with, for example, the bones of the leg.